The diagnosis of bronchiolitis is based mainly on the patient's medical history and physical examination. However, in severe cases, a further evaluation including chest X-ray (CXR) may be necessary. At present, lung ultrasound (LUS) is not included in the diagnostic work-up of bronchiolitis. This study aimed to compare the diagnostic accuracy of LUS and CXR in children with bronchiolitis, and to evaluate the correlation between clinical and ultrasound findings. Only patients with a diagnosis of bronchiolitis, who had undergone a CXR, were enrolled in the study. Fifty-two infants underwent LUS and CXR. LUS was also performed in 52 infants without clinical signs of bronchiolitis. LUS was positive for the diagnosis of bronchiolitis in 47/52 patients, whereas CXR was positive in 38/52. All patients with normal LUS examination had a normal CXR, whereas nine patients with normal CXR had abnormal LUS. In these patients, the clinical course was consistent with bronchiolitis. We found that LUS is a simple and reliable tool for the diagnosis and follow-up of bronchiolitis. It is more reliable than CXR, can be easily repeated at the patient's bedside, and carries no risk of irradiation. In some patients with bronchiolitis, LUS is able to identify lung abnormalities not revealed by CXR. Furthermore, there is a good correlation between clinical and ultrasound findings. Given the short time needed to get a US report, this technique could become the routine imaging modality for patients with bronchiolitis.